With the advancement of mobile communication terminal functions, the terminals are more than simple wireless communication devices and may include functions such as a camera function and a multimedia reproduction function in addition to a wireless communication function. Such camera and multimedia reproduction functions are advanced functions that have been gradually popularized because of display unit enhancements and high-speed communication. A terminal having a high-resolution camera and a liquid crystal display (LCD) screen is already in general use.
In addition, a terminal providing a PTT service has been developed. Like a walkie-talkie service, the PTT service is a half-duplex communication method, wherein only one party may use the communication channel at a given time, and provides One-to-One or One-to-Many communication service.
Full-duplex communication allows two-way voice communication without any distinction between a speaker and a listener. However, in half-duplex communication such as the PTT service, an obvious distinction exists between a speaker and a listener, and only one user can be a speaker at a given time. A system supporting a terminal PTT service in accordance with the related art is shown and described with in FIG. 1.
FIG. 1 shows how signals flow in a terminal supporting PTT service in accordance with the related art, wherein the system supporting a PTT service includes: a sender's terminal 10 for sending voice data by using a PTT key mounted on the terminal; a Session Initiation Protocol (SIP) Proxy server 20 for transmitting the sent voice data; and a recipient's terminal 30 for receiving the transmitted voice data.
The operation of the terminal supporting a PTT service in accordance with the related art having such construction will be described as follows. A PTT key is mounted on the terminal in order to request the floor, and the user requests the floor by pressing the mounted PTT key. In other words, said sender's terminal 10 sends an INVITE message that the user requests the floor to the SIP proxy server 20.
The SIP proxy server 20 receives the INVITE message, checks a position of the corresponding recipient's terminal 30 on the basis of the received INVITE message, and sets up a session between the recipient's terminal 30 and the sender's terminal 10 using SIP. If the SIP proxy server 20 cannot set up a session between the recipient's terminal 30 and the sender's terminal 10, for example, when the recipient's terminal 30 is in a shadow area or a reception rejection state, the SIP proxy server sends an error message to the sender's terminal 10. The sender's terminal 10 then cannot send a voice message to the recipient's terminal 30.
If a session between the recipient's terminal 30 and the sender's terminal 10 is established, the sender's terminal 10 sends the voice message to the SIP proxy server 20, the SIP proxy server 20 sends the sent voice message using Real Time Protocol (RTP) to the session-established recipient's terminal 30.
However, since the terminal supporting a PTT service in accordance with the related art sends a voice message only to the session-established recipient's terminal, there is a problem that the system cannot transmit the voice message to the recipient's terminal having no session established with the sender's terminal.